This invention relates to a selector device for use in the casting of single crystal objects.
In recent years casting single crystal objects has become a practical proposition, and the potential advantages of such objects in difficult environments such as the turbine region of gas turbine engines have lead to considerable effort being put into the development of the casting process.
The technique most usually adopted to produce a single crystal casting involves the use of a chill, a mould having initiating and main cavities, a selector device and a directional solidification furnace. The casting process then involves pouring a charge of molten metal to fill the whole of the mould including both cavities and the selector device, and causing the metal to solidify in such a way that the solidification front travels uniformly from the chill and through the initiating cavity, the selector and the main cavity in order.
The directional solidification of the metal from the chill and into the initiating cavity gives rise initially to a columnar grain structure in which the grains of the solidified metal extend unidirectionally away from the chill and toward the selector. When the solidification front reaches the selector, which comprises a relatively narrow passage of curvilinear or helical shape, not all of the columnar grains can continue to grow, and the selector passage is chosen so that only one grain or crystal succeeds in growing through the passage and into the main cavity.
On debouching (emptying) into the main cavity, the single crystal alone will continue to grow as long as the transition from the selector passage into the main cavity is sufficiently smooth and as long as no other grain initiating irregularities are present. The metal in the main cavity, which is arranged to have the form of the required cast object, is thus caused to solidify as a single crystal of the desired form.
One problem with the apparatus used for this casting process has arisen because of the narrow, curvilinear shape required for the selector passage. The conventional way of providing this passage has been to produce a wax duplicate of the required passage, and to form a shell round this wax duplicate in the normal manner used for lost-wax castings. The shell can then be attached to the initiating and main cavities, or alternatively the wax can be attached to the wax patterns which form the initiating and main cavities and the vanes can be shelled as an integral whole.
Although this technique produces quite satisfactory internal cavities and selector passage, that part of the wax pattern which will eventually define the passage is inevitably narrow and relatively weak and this weakness is compounded by the convoluted form of the passage. Consequently this part of the pattern is prone to damage, in the shelling process and in any handling. It will also be noted that producing the duplicate of the passage is not easy, since this cannot be carried out using a simple two-part die.
Attempts have been made to solve these problems, using reinforcing means for the selector passage shell or support means for the rest of the shell mould. However, these have tended to be complex, increasing the cost of what is already a relatively expensive process.
The present invention provides a selector of simple form which is relatively strong.